In a traditional telephone network, ringback is typically an audible tone sequence presented during call setup to a caller to signal that the called party's phone is being alerted of the caller's incoming call to the called party. Upon hearing ringback, the caller may generally assume that the called party is receiving a corresponding, concurrent (or nearly concurrent) alert, such as an audible ringtone sequence. The ringback usually continues until either the called party (or a call-handling function, such an answering machine) answers the call or the caller hangs up before the call is answered. The time within the call setup during which ringback is played is typically referred to as the ringback period.
The model for ringback has evolved similarly in both circuit-switched telephony with out-of-band signaling and services (Signaling System 7 (SS7) and Advanced Intelligent Network (AIN), for example), and packet-based telephony, such as Voice over Internet Protocol (VoIP). Ringback tone in these types of systems is typically stored as an audio data file on a network entity, such as a network server, and played out to a caller's phone during call setup when the entity receives a signal, from a terminating switch for example, indicating that the called party's phone is being alerted.
The server-based model of ringback tone storage/playout can support multiple ringback tones, and lends itself to a variety of schemes for customization of ringback tones. An example of customization of ringback tone is a user feature that allows a subscriber in a carrier or service provider network to select one or more ringback tones that will be presented to callers to that user. The carrier or service provider may in turn charge a fee to the subscriber for access to the feature.